Paper-based chemical assay devices include a paper substrate, wax that forms fluid channels and other fluid structures in the paper, and one or more reagents. Common examples of paper-based chemical assay devices include biomedical testing devices that are made of paper and perform biochemical assays and diagnostics in test fluids such as blood, urine and saliva. The devices are small, lightweight and low cost and have potential applications as diagnostic devices in healthcare, military and homeland security to mention a few.
FIG. 7A and FIG. 7B depict the prior art processes for melting wax that is formed on a paper substrate in a reflow oven. The melting process is required for the wax to penetrate into the paper instead of remaining in a layer on the surface of the paper. In FIG. 7A, a reflow oven heats a paper substrate with solidified wax to a temperature of approximately 150° C. The entire paper and the wax are heated to the same temperature in an isotropic manner. As depicted in FIG. 7B, the wax melts and spreads both into the porous paper and across the surface of the paper in a roughly uniform manner. The prior art reflow oven cannot control the direction of flow for the melted wax, and the melted wax tends to spread across the surface of paper to a greater degree than is desired. In a biomedical testing device, the wax is formed in lines and other structures that act as barriers and channels to fluids that diffuse through the paper substrate. The uncontrolled spread of the wax presents difficulties in forming the barriers and liquid channels with precise shapes. Consequently, improvements to the control the flow of a hydrophobic material that is deposited on a porous substrate would be beneficial.